Most gamma cameras presently use what is known as Anger mathematics or Anger calculations, as shown in U.S. Pat. No. 3,011,057 to determine the location of the event at what may be termed a first approximation. Subsequently correction factors are used to correct the Anger calculation determined location.
Correction factors include, for example, dislocation correction factors obtained by using a flood field in conjunction with a gamma radiation shield having perforations therein at known locations. Correction factors are computed for determining the distance between the known locations of the perforations and the image locations of the perforations. These calculations are used for correcting the locations of the events as they occur during the clinical use of the gamma camera.
In practice it is advisable to recalibrate the correction factors periodically; that is, to perform the flood field dislocation calibration scheme periodically because the previously obtained dislocation correction factors are no longer accurate due to drift in the hardware components. Accordingly, the correction factors while presumed correct at the beginning of the periodic term, eventually become relatively incorrect unless the electronics are "retuned".
In addition to the inconvenience, the dislocation correction calibration takes time and equipment. Further the actual dislocation correction, while done on the fly, still requires a memory for storing the correction factors and a mathematic performing unit to do calculations utilizing the correction factors. Hence the need to recalibrate requires more hardware and/or software in the gamma cameras. Since the widespread adaption of the Anger camera calculation systems, other means and methods have been sought to obtain a more accurate method of precisely locating events as they occur.
Statistical methods have been suggested for locating the events on the gamma camera for imaging purposes. However in the past these statistical methods have required vast memories, intensive computation and extra equipment for the actual calculations; so that, the statistical methods have proved impractical.